L. N. Davy in U.S. Pat. No. 4,330,753 issued 18 May 1982 and entitled "METHOD & APPARATUS FOR RECOVERING A SIGNAL FROM A CHARGE TRANSFER DEVICE" describes the synchronous detection of CCD imager output signals at a harmonic of the output register clocking frequency to recover reduced-noise video. In U.S. Pat. App. Ser. No. 525,491 filed Aug. 22, 1983, entitled "IMPROVED RECOVERY OF SIGNAL FROM CHARGE TRANSFER DEVICES," and assigned to a common assignee RCA Corporation (substantial portions of which application appear in the detailed description of the drawing to follow), the present inventor describes the use of synchronous detectors of a sample-and-hold type being used to recover video from CCD output signal in which the lower portion at least of the baseband is suppressed.
The d-c restoration of CCD imager output signals has heretofore been done as follows. The line register used for parallel-to-serial conversion of image sample format at the output of the CCD imager has its clocking continued for a time after the image samples transferred in parallel into the line register have been serially clocked out of the line register. The continued clocking of the line register provides samples of wells which are empty except for the very small dark current accumulation in the line register, and these samples are used to establish black level. D-C restoration is accomplished by clamping of the video output signal to the reference dc level each line, during the time black level is established. Since the periods of time between the picture portions of line trace are of fixed duration, this method of d-c restoration requires limitation of the length of time that empty wells are clocked out of the imager in order to establish black level. This time has to be limited to a fraction of line retrace interval, to allow time for the ensuing transfer of a row of charge packets in parallel from the field storage register to the output line register during the remainder of line retrace interval.
When synchronous detection of harmonics of CCD output register clocking frequency has been employed, the above-described method of d-c restoration has continued to be used, with clamping taking place after the synchronous detector. However, d-c restoration of this type is affected by noise occurring during the clamping interval; and the error in d-c restoration is retained through the remainder of the line, giving rise to streak noise extending across lines of the picture reconstituted from the video originating from CCD imager. The streak noise can be suppressed by extending the time constant associated with d-c restoration to well over a field time, but it is well-known that a time-constant longer than ten lines or so is to be avoided for other reasons. The streak noise problem becomes increasingly noticeable at low ambient light levels, where the CCD imager is provided only a few tens of electrons charge per image sample, requiring the amplification of the imager output signal to be increased. The noise of the amplifier is not only added to the CCD imager output signal during image read-out times, but is also added during the times clamping is done to restore d-c. This internally-generated amplifier noise added to the variably amplified clock noise provides the noise that afflicts d-c restoration.
This prior art method of d-c restoration is also disadvantageous in that the lengthening of the time the output line register is clocked each line encroaches upon the line retrace interval, reducing the time available for line by line transfer in the preceding field storage register. In larger imagers where the gate electrodes are wider in the field storage register, so their associated capacitances are larger, shortening of the time available for line by line transfer in that register reduces the modulation transfer function (MTF) appreciably. That is, the complete charge packet describing an image sample fails to be clocked forward; and a smear results, reducing resolution in the direction perpendicular to line scan.
When the low-frequency components of recovered video signal are developed solely by synchronous detection of the CCD output signal at a harmonic of output line register clocking frequency, the suppression of lower-frequency baseband components in the CCD imager output signal results in a signal of zero average value being supplied to the synchronous detector. Therefore, there is no direct-voltage average step introduced into the synchronous detector output as input signal to the synchronous detector is discontinued or resumed. This zero average value signal, while not at black level for the baseband spectrum, is at black level for the sideband spectrum around harmonics of the clocking frequency of the CCD imager output register.
Where the lower frequency components of the baseband spectrum are suppressed in the CCD imager output signal prior to its synchronous detection, leaving the higher frequency components of the baseband spectrum for video peaking, as described in U.S. Pat. App. Ser. No. 525,491, one finds there is a relatively-small-amplitude average step as the input signal to the synchronous detector is discontinued or resumed during the time empty well samples are clocked out of the line register at the CCD imager output. The relatively-small-amplitude average step is presumably due to synchronous detection of clock noise and is not large enough to affect d-c restoration appreciably. Furthermore, the error introduced is invariant with signal variation and can be compensated against in the later video circuitry. Remnant high frequency components of the CCD imager output signal baseband spectrum are asynchronous to clocking frequency harmonics, so do not give rise to d-c shifts in synchronous detector output signal. As in the previous case where the baseband spectrum is completely suppressed, zero average value signal at the input of the synchronous detector is at black level for the sideband spectrum around harmonics of the clocking frequency of the CCD imager output register.